Electroluminescent lamp having a moisture resistant vitreous light-transmitting material which includes 4 to 8% lif



1966 CHIKARA HIRAYAMA 3,290,535

ELECTROLUMINESCENT LAMP HAVING A MOISTURE RESISTANT VITREOUSLIGHT'TRANSMITTING MATERIAL WHICH INCLUDES 4 TO 870 LiF Filed Oct. 31.1961 INVENTOR.

CHIKARA HIRAYAMA.

HUU/PNEY 3,290,535 ELEQTRGLUMKNESCENT LA HAVING A MQllS- TURE RESISTANTVITREQUS MIGHT-TRANS- METTKNG MATERIAL WHlhCH INCLUDES 4 T 8% LilFChikara Hirayarna, Murrysville, lia., assignor to Westinghouse ElectricCorporation, Pittsburgh, Pa, 21 corporation of Pennsylvania Filed Get.31, 1961, Ser. No. 149,119 9 Claims. (Cl. 313-108) This inventionrelates to electroluminescent devices and, more particularly, toceramic-type electroluminescent devices and light-transmitting, vitreouscover coats therefor.

The phenomenon of electroluminescence is now well known and such deviceshave been marketed commercially. in one construction for such devices,finely divided phosphor is embedded in a glass or ceramic material inthe form of a layer and the energizing electric field is applied acrossthis layer to produce light. The phosphor which is generally used iscopper-activated zinc sulfide. Zinc sulfide and other knownelectroluminescent phosphors display a relatively poor maintenance ofinitial light output in the presence of moisture. In other words, in thepresence of moisture, the light output drops more rapidly duringoperation than when such devices are operated in a very dry atmosphere.

In order to inhibit this drop in light output, it has been customary toplace a moisture-protecting layer over the moisture-previous portions ofthe device. In the case of a ceramic-type electroluminescent device, aceramic moisture-protecting layer must meet fairly stringentrequirements. In explanation, in the assembly of such ceramic-typedevices, the phosphor-containing ceramic layer is first formed over anenameling iron substrate by firing at approximately 650 C. Over thephosphor-ceramic layer is formed the light-transmitting electrode, suchas a layer of tin oxide. A protective or cover coat layer is then formedover this light-transmitting electrode. In order to prevent damaging thefirst-applied phosphor-dielectric layer, the cover coat must be formedat a temperature of less than about 650 C. In addition, the vitreousmaterial of which this coat is formed must be sufficiently fluid at arelatively low temperature that it will flow readily and form a uniform,continuous thin layer in a relatively short period of time. Also, thecoefficient of expansion desirably should approach that of thephosphor-containing vitreous layer. As an additional requirement, theso-called cover coat layer, when cooled, should be highly resistant topenetration -'by moisture, in order to protect the operative portions ofthe electroluminescent device. A good vitreous, light-transmitting covercoat which meets all of these requirements has not been available.

It is the general object of this invention to avoid and overcome theforegoing and other difficulties of and objections to prior artpractices by the provision of a ceramic-type electroluminescent devicehaving a vitreous, light-transmitting cover coat which is extremelyresistant to penetration by moisture.

It is another object to provide a vitreous, light-transmitting materialwhich when in a finely divided state can be heated to a temperature lessthan 650 C. to readily form a continuous layer, which layer when cooledis highly resistant to penetration by moisture.

The aforesaid objects of the invention, and other objects which willbecome apparent as the description proceeds, are achieved by providing amodified bariumborate glass system which contains appreciable amounts ofmagnesium oxide, zinc oxide and lithium fluoride. Limited amounts ofother glass ingredients, namely calciurn oxide, sodium oxide andaluminum oxide, are also desirably included in the glass. In order toachieve the desired effects, the relative proportions by weight of theindividual glass constituents must be carefully balanced and relativeweight percentage ranges are included.

For a better understanding of the invention, reference should be had tothe accompanying drawing wherein the sole figure illustrates aperspective view of a ceramic-type electroluminescent device, an end ofwhich is shown in section, and which device is provided with thevitreous, light-transmitting cover coat of the present invention.

With specific reference to the form of the invention shown in thedrawing, the electroluminescent device 10 generally comprises aconducting substrate 12 over which is applied a phosphor-dielectriclayer 14. A light-transmitting electrode layer 16 is carried over thelayer 14 and the vitreous, light-transmitting cover coat 18 of thepresent invention is carried over the electrode layer 16. Lead-in wires20 connect the electrodes 12 and 16 to a source of AC. energizingpotential.

As a specific example, the substrate 12 is formed of conventionalenameling iron and the phosphor-dielectric layer 1 1 is formed of finelydivided, copper-activated zinc sulfide phosphor embedded in glassdielectric material in the approximate proportions of one part by weightof phosphor per two parts by weight of glass dielectric. For a specificexample of a suitable glass dielectric for embedding the phosphor,reference is made to copending application S.N. 78,156, filed December23, 1960, and owned by the present assignee, now Patent No. 3,073,982.The electrode layer 16 is formed of tin oxide and the use of such anelectrode layer is well known in the electroluminescent art.

The foregoing electroluminescent device is subject to considerablemodification. For example, any conventional electroluminescent phosphorcan be substituted for the zinc sulfide as specified. The relativeproportions by Weight of phosphor and dielectric are not critical andcan be varied. In addition, other light transmitting electrode materialscan be substituted for the preferred tin oxide.

The phosphor-dielectric layer 14, together with surrounding electrodelayers 12 and 16, constitute the operative portion of the device. Theenameling iron electrodesubstrate 12 is quite impervious to moisture,but the electrode layer 16 is readily pervious to moisture and requiresprotection. In accordance with the present invention, the specific covercoat layer 18 of the present invention is provided over the electrodelayer 16 and preferably extends beyond the electrode layer 16 and ontothe enameling iron substrate 12.

The vitreous cover coat 18 of the present invention consists essentiallyof the following constituents in about the following relativepercentages by weight: B 0 2338%; SiO 1-15%; BaO, 10-52%; ZnO, 527%;LiF, 47%;

Because of the relatively high percentage of boric oxide, the presentglass can be generally categorized as a boratetype glass. The relativelylarge percentage of barium oxide, together with the zinc oxide, providethe glass with a relatively low sag point, as will be explained ingreater detail hereinafter. The magnesium oxide and lithium fluorideprovide the glass with good moisture resistant characteristics. It isalso desirable to include some aluminum oxide in the glass up to themaximum amount as specified, in order to improve the surface hardness ofthe formed cover coat layer to prevent scratching. The calcium oxide, ifused, also imparts moisture-resistant characteristics to the glass andthe sodium oxide acts as a flux to facilitate initial smelting.

The relative proportions by weight of the constituents which comprisethe glass must be carefully controlled if the desired characteristicsare to be obtained. For ample. This causes the frit to form a smooth,continuous, example, if the relative amounts of calcium andmagsubstantially transparent layer having a thickness of 5 nesium oxidesare excessive, the softening and sag temmils.

peratures of the glass will be too high, If too much The thickness ofthe present cover coat layer 18 is lithium fluoride is included in theglass, the light-trans- 5 not critical but preferably is in the range offrom 3 mils mitting characteristics thereof will be impaired. Sufiito 8mils. Thicker cover coat layers may be difiicult to cient boric oxideand silica are required in order to provide apply and overly thin covercoat layers will limit the the glass with a low flow temperature andsimilarly, it is moisture protection properties of this layer.

necessary to include the barium oxide and zinc oxide with- The covercoats of the present invention have a coefin indicated relativeproportions by weight in order that 10 ficient of expansion which isjust slightly less than th e the melting temperature of the glass willbe sufiiciently coeflicient for the phosphor-dielectric layer 16. T11181s low. desirable since residual strains in the cover coat are ninitially forming the glass, a batch composition slightly compressive innature which assists in forming containing the ingredients in the properproportions is a very g glass y The eeetheiehts tor the P melted in aporcelain crucible in an electric furnace at a ent Cover Coat v y from75 t0 This p r re f fr m 900 C. to 1100 C. and preferably range can beextended or varied, depending on the coeffrom 900 C. to 1000 C. Thebatch is heated for apficient of the phosphor-dielectric layer.proximately forty-five minutes and the molten compo- In actual tests,lamps which incorporated the Covet sition is then poured onto astainless steel plate which eeilt 0f the Present invention were Operatedat 60 y is externally cooled with cold water, This causes the 120 Voltsin a Chamber at a relative humidity of 98% glass to cool rapidly andshatter and it is thereafter and a temperature of These lamps Operatedfor crushed and ball milled with a suitable vehicle such as thirty y andlonger before taihlre- Under Such acetone to a \mesh size of 200 orfiner, Exam l of ditions of operations, the best normal life which couldspecific batch compositions and their sag points are given be expectedfrom lamps which were protected with other in the following Table I,with the relative amounts of the known cover coat layers wasconsiderably less and a batch constituents given in terms of theequivalent oxide, representative life was approximately two to threedays. except for the lithium fluoride. The electroluminescent lamp, asshown in the drawing,

TABLE I.MOLE PERCENT, BATCH COMPOSITION Batch B 0 8101 B210 Z110 LiF MgOCaO Na O A120; E E -Y as 10 20 10 15 5 s 466 In explanation of the termsag point in describing is subject to some modification in that anadditional layer the properties of glass, this is the temperature atwhich of semiconductor material may also be lncluded between the glasshas a viscosity of approximately 10 poises, the device electrode, asdisclosed in the aforesaid co- Normally the sag point will beapproximately 75 C. pending application S.N. 78,156, filed December 23,higher than the softening temperature of the glass. A 1960. In addition,intermediate layers may be apphed 10W sag point is indicative that aglass .frit will readily over the electrode layer 16 before the covercoat of the form into a continuous mass or layer when the glass isPresent invention is pp Alternatively, the Phosphor h ated to arelatively low temperature. can be formed as a continuous thin film, asdisclosed in The ingredients used in initially mixing the foregoingeoperldihg application 1 filed September batches were boric oxide,silica, barium carbonate, zinc 1959, and Owned y the Present asstghee,how Patent oxide, lithium fluoride, magnesia, calcium carbonate, so- InSueh ease, Separate ihergehie matedium carbonate and aluminum oxide, P fbl th rial layer can be included between the dev1ce electrodesingredients are dry mixed in a ball mill for at least of the P P film pse can he Sandwiched between one hour before smelting in the manner asindicated. the device eleetrodes- The Cover Coat of the Present In thefollowing Table II are listed the constituents of invention is thenPlaced Over the moisture-perviohs eleethe smelted batches, in terms ofweight percents of the trodes to Protect the device final compo ition,It will be recognized that the ob ects of the present invention has beenachieved by providing a ceramic-type electroluminescent device which hasimproved performance and maintenance of lumen output. In addition, therehas been provided a vitreous, light-transmitting material which, when infinely divided state, can be TABLE II.WEIGIIT PERCENT, BATCH COMPOSITIONBatch B10 8102 B210 ZnO LiF MgO CaO NagO A1 0;

22.6 24.0 7.7 6.0 heated to a temperature less than 650 C. to readilyform I u e 5? 2'3 a continuous layer and which layer, when cooled, 1slngh- 22. e 17. 8 7. 6 3. 0 1y impervious to penetration by moisture.

While at best embodiments of the invention have been illustrated anddescribed in detail, it is to be particular- To form anelectroluminescent lamp such as shown in ly understood that theinvention is not limited thereto the drawing, the phosphor-dielectriclayer 14 and the elecor thereby. trodes 12 and 16, which form theoperative portion of the I claim as my invention: 4 device 10, are firstfabricated in conventional fashion. 1. A vitreous, light-transmittingmaterial which in Thereafter the cover coat glass of the presentinvention finely divided state can be heated to a temperature of less ismixed in finely divided form with an 80% water-20% than 650 C. toreadily form a continuous layer and ethanol vehicle to form a slurrywhich is sprayed onto the which formed layer when cooled is highlyresistant to electrode layer 16. Alternatively, a silk-screen processpenetration by moisture, said vitreous material consisting can be usedto apply the glass frit. Thereafter, the apessentially of from about 23to 38% by weight B 0 from plied frit is fired at a temperature ofbetween 600 and about 1 to 15% by weight SiO from about 10 to 52% 650C., for example, for from 5 to 10 minutes, for exby weight BaO, fromabout 5 to 27% by weight ZnO,

5 from about 4 to 7% by weight LiF, from about 2 to 8% by weight MgO,from about to by weight Na O, from about 0 to 8% by weight CaO, and fromabout 0 to 6% by weight A1 0 2. A vitreous, light-transmitting materialwhich in finely divided state can be heated to a temperature of lessthan 650 C. to readily form a continuous layer, and which formed layerwhen cooled is highly resistant to penetration by moisture, saidvitreous material consisting essentially of about 28% by weight B 0about 8% by weight SiO about 32% by weight BaO, about 22% by weight ZnO,about 5% by weight LiF, and about 5% by weight MgO.

3. A vitreous, light-transmitting material which in finely divided statecan be heated to a temperature of less than 650 C. to readily form acontinuous layer, and which formed layer when cooled is highly resistantto penetration by moisture, said vitreous material consistingessentially of about 31% by weight B 0 about 4% by weight SiO about 23%by weight BaO, about 24% by weight ZnO, about 8% by weight LiF, about 6%be weight MgO, and about 4% by weight CaO.

4. A vitreous, light-transmitting material which in finely divided statecan be heated to a temperature of less than 650 C. to readily form acontinuous layer, and which formed layer when cooled is highly resistantto penetration by moisture, said vitreous material consistingessentially of about 35% by weight B 0 about 4% by weight S102, about22% by weight BaO, about 23% by weight ZnO, about 6% by weight LiF,about 6% by weight MgO, and about 4% by weight CaO.

5. A vitreous, light-transmitting material which in finely divided statecan be heated to a temperature of less than 650 C. to readily form acontinuous layer, and which formed layer when cooled is highly resistantto penetration by moisture, said vitreous material consistingessentially of about 35% by weight B 0 about 9% by weight SiO about 22%by weight BaO, about 18% by weight ZnO, about 6% by weight LiF, about 6%by weight MgO, and about 4% by weight Na O.

6. A vitreous, light-transmitting material which in finely divided statecan be heated to a temperature of less than 650 C. to readily form acontinuous layer, and which formed layer when cooled is highly resistantto penetration by moisture, said vitreous material consistingessentially of about 36% by weight B 0 about 13% by weight SiO about 22%by weight BaO, about 18% by weight ZnO, about 8% by weight LiF, andabout 3% by weight MgO.

7. In combination with an electroluminescent device 50 the operativeportion of which comprises spaced electrodes, one of which islight-transmitting, with electroluminescent phosphor embedded inlight-transmitting vitreous material included between said spacedelectrodes, the improvement which comprises, a vitreouslight-transmitting layer over said light-transmitting electrode andexterior to the operative portion of said device, said vitreous layerconsisting essentially of the following constituents within about thespecified relative proportions by weight: 23% to 38% B 0 1% to 15% SiO10% to 52% BaO, 5% to 27% ZnO, 4% to 7% LiF, 2% to 8% MgO, 0 to 5% Na O,0 to 8% CaO, and 0 to 6% A1 0 8. In combination with anelectroluminescent device the operative portion of which comprisesspaced electrodes, one of which is light-transmitting, with phosphorembedded in light-transmitting vitreous material included between saidspaced electrodes, the improvement which comprises, a vitreouslight-transmitting layer have a thickness of from about 3 to 8 mils oversaid light-transmitting electrode and exterior to the operative portionof said device, said vitreous layer consisting essentially of thefollowing constituents within about the specified relative proportionsby weight: 23% to 38% B 0 1% to 15% SiO 10% to 52% BaO, 5% to 27% ZnO,4% to 7% LiF, 2% to 8% MgO, 0 to 5% Na O, 0 to 8% CaO, and 0 to 6% A1 09. In combination with an electroluminescent device the operativeportion of which comprises spaced electrodes, one of which islight-transmitting, with phosphor embedded in light-transmittingvitreous material included between said spaced electrodes, theimprovement which comprises, a vitreous light-transmitting layer havinga thickness of about 5 mils over said light-transmitting electrode andexterior to the operative portion of said device, said vitreous layerconsisting essentially of the following constituents Within about thespecified relative proportions by weight: about 36% by weight B 0 about13% by weight SiO about 22% by weight BaO, about 18% by weight ZnO,about 8% by weight LiF, and about 3% by weight MgO.

References Cited by the Examiner UNITED STATES PATENTS 2,887,402 5/1959Ballard 313108.1 2,922,912 1/1960 Miller 313108.1 2,965,784 12/1960Hoffman 313108.1 2,993,001 7/1961 Shonebarger 106-54 X 3,005,721 10/1961Cerulli 10654 3,005,722 10/1961 Cerulli 106-54 3,095,311 6/1963 VonWranau 10654 JAMES W. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, DAVID J. GALVIN, C, R.

CAMPBELL, R. JUDD, Assistant Examiners.

7. IN COMBINATION WITH AN ELECTROLUMINESCENT DEVICE THE OPERATIVEPORTION OF WITCH COMPRISES SPACED ELECTRODES, ONE OF WHICH ISLIGHT-TRANSMITTING, WITH ELECTROLUMINESCENT PHOSPHOR EMBEDDED INLIGHT-TRANSMITTING VITREOUS MATERIAL INCLUDED BETWEEN SAID SPACEDELECTRODES, THE IMPROVEMENT WHICH COMPRISES, A VITREOUSLIGHT-TRANSMITTING LAYER OVER SAID LIGHT-TRANSMITTING ELECYTRODE ANDEXTERIOR TO THE OPERATIVE PORTION OF SAID DEVICE, SAID VITREOUS LAYERCONSISTING ESSENTIALLY OF THE FOLLOWING CONSTITUENTS WITHIN ABOUT THESPECIFIED RELATIVE PROPORTIONS BY WEIGHT: 23% TO 38% B2O3, 1% TO 15%SIO2, 10% TO 52% BAO, 5% TO 27% ZNO, 4% TO 7% FIF, 2% TO 8% MGO, 0 TO 5%NA2O, TO 0 TO 8% CAO, AND 0 TO 6% AL2O3.